In submerged chiller refrigerating applications, the outside of a heat transfer tube is normally submerged in a refrigerant to be boiled, while the inside conveys liquid, usually water, which is chilled as it gives up its heat to the tube and refrigerant. In a boiling application such as a refrigerating application, it is desirable to maximize the overall heat transfer coefficient.
In order to maximize the heat transfer coefficient, it is known to make modifications to the outside surface of a heat transfer tube in order to take advantage of the phenomenon known as “nucleate boiling”. According to one example, the outer surface of a heat transfer tube may be modified to produce multiple pockets (i.e., cavities, openings, enclosures, boiling sites, or nucleation sites) which function mechanically to permit small vapor bubbles to be formed therein. The vapor bubbles tend to form at the base or root of the nucleation site and grow in size until they break away from the outer surface. Upon breaking away, additional liquid takes the vacated space and the process is repeated to form other vapor bubbles. In this manner, the liquid is boiled off or vaporized at a plurality of nucleate boiling sites provided on the outer surface of the metallic tubes.
According to one example, the external enhancement is provided by successive cross-grooving and rolling operations performed after finning of the tubes. The finning operation, in a preferred embodiment for nucleate boiling, produces fins while the cross-grooving and rolling operation deforms the tips of the fins and causes the surface of the tube to have the general appearance of a grid of generally flattened blocks. The flattened blocks are wider than the fins and are separated by narrow openings between the fins. The roots of the fins and the cavities or channels formed therein under the flattened fin tips are of much greater width than the surface openings so that the vapor bubbles can travel outwardly through the cavity and through the narrow openings. The cavities and narrow openings and the grooves all cooperate as part of a flow and pumping system so that the vapor bubbles can readily be carried away from the tube and so that fresh liquid can circulate to the nucleation sites.
It is desirable to use heat transfer tubes having surface enhancements in the form of nucleation sites in other types of heat transfer applications where maximizing the overall heat transfer coefficient is important.